Various iron alloys have been prepared through the ages. One of the most common iron alloys is steel, which generally comprises 0.2 to 2.1% wt (percent by total weight) of carbon in addition to iron. Carbon is added as a hardening agent, i.e. its addition renders steel harder than iron.
Iron and steel tend to oxidize under many practical conditions, thereby forming iron or iron-chromium oxides. Oxidation of iron or steel work pieces is often undesirable because of the resulting degradation of mechanical properties, and because of a degraded visual appearance.
In order to reduce the susceptibility of iron alloys to corrosion, further elements are commonly introduced to the alloys. One variety of iron alloys is stainless steel which comprises at least 10.5% chromium by weight. This chromium content corresponds to the minimum amount required to form a passive layer on the steel surface under atmospheric conditions. In addition to chromium, further elements frequently found in stainless steels are Ni and Mo. Their purpose is to further improve corrosion resistance.
Stainless steels are characterized by a very thin and stable surface layer called a passive layer. The passive layer is rich in chromium oxide and generally has a thickness of 1 to 5 nm. It consists of approximately 70% wt chromium oxide and 30% wt iron oxide. Chromium oxide is thermodynamically stable and chemically inert under atmospheric conditions, and the chromium oxide rich passive layer is non-permeable for most oxidizing agents which might initiate corrosion. However, corrosion of this passive layer occurs nevertheless under specific conditions which commonly occur in the pharmaceutical industry. In particular, it is frequently observed in the pharmaceutical industry that stainless steel surfaces tend to develop a red to black thick surface layer (rouging) upon contact with hot (above 50° C.) water (vapour or liquid) having a conductivity less than 1 μS/cm2. For instance, the surfaces of Water-For-Injection equipment inspected after 6 to 18 months often show the so-called rouging effects, i.e. the presence of an increased concentration of iron oxide on the surface. Apart from iron oxide, mixed metal oxides are also present such as iron-chrome-nickel mixed metal oxides.
The cause of rouging is not well known yet. Rouging presents a danger in the pharmaceutical industry since it leads to the release of heavy metal oxide particles from the stainless steel surface. This entails the danger of undesired contamination of pharmaceuticals with heavy metal particles, and therefore rouging negatively affects the purity and quality of produced and/or processed products. Furthermore, rouging increases the micro-roughness of affected stainless steel surfaces.
Because of the deleterious effects of rouging, great efforts have been directed to rouging removal. For example, WO2009095475 describes a method for the treatment of thick oxidic iron layers (rouging) on stainless steel (AISI 316L) surfaces. The method makes use of a reductive solution consisting of a freshly prepared 0.25% solution of solid sodium dithionite in water which additionally comprises 0.5% 2-phosphonobutane-1,2,4-tricarboxylic acid sodium salt (PBC-Na4), 0.25% tetrasodiumiminodisuccinate, 0.05% potassium oxalate, and 0.1% of sodium hydrogen carbonate/carbonic acid buffer. The use of this reductive solution by contacting the oxidized stainless steel surface with it is claimed to completely remove the rouging from the stainless steel surface. A presentation from the inventor of WO2009095475 (rouging and derouging of stainless steel, Marc Vernier, USA, October 2009, retrieved from the website www.ultracleanep.com) clearly shows that the aspect of the stainless steel obtained after derouging often still presents a black veil. Furthermore, the use of sodium dithionite is restricted to oxygen-free conditions since sodium dithionite oxidizes easily. As a result, the cleaning needs to be performed under an inert atmosphere which is not convenient. In addition, the aforementioned method merely provides a curative treatment for rouging. It would be more desirable to prevent rouging altogether.